This invention relates to the use of autodepositing aqueous liquid compositions that are both dispersions and solutions in water. By mere contact with these autodepositing liquid compositions, active metal surfaces can be coated with an adherent polymer film that increases in thickness the longer the time of contact, even though the aqueous liquid composition is stable for a long time against spontaneous precipitation or flocculation of any solid phase, in the absence of contact with active metal. (For the purposes of this specification, the term "active metal" is to be understood in its broadest sense as including all metals and alloys more active than hydrogen in the electromotive series, or, in other words, a metal is which is thermodynamically capable of dissolving to produce dissolved cations derived from the metal, with accompanying evolution of hydrogen gas, when contacted with an aqueous solution of a non-oxidizing acid in which the activity of hydrogen ions is 1.00 equivalent per liter.) Such liquid compositions are denoted in this specification, and commonly in the art, as "autodeposition" or "autodepositing" compositions, dispersions, emulsions, suspensions, baths, solutions, or a like term. Autodeposition is often contrasted with electrodeposition, which can produce very similar adherent films but requires that the surface to be coated be connected to a source of direct current electricity for coating to occur.
More particularly, this invention relates to autodeposition in which the adherent polymer film that forms includes as its predominant organic constituent polymers that include substantial amounts of chlorine atoms, as more specifically detailed in U.S. Pat. No. 5,352,726 of Oct. 4, 1994 to Hall, the entire disclosure of which, except to any extent that it may be contrary to any explicit statement herein, is hereby incorporated herein by reference.
The coating formed while a metal substrate is immersed in an autodeposition bath is wet and fairly weak, although sufficiently strong to maintain itself against gravity and moderate spraying forces. In this state the coating is described as "uncured". To make an autodeposition coated object suitable for normal practical use, the uncured coating is dried, usually with the aid of heat. The coating is then described as "cured".
The present invention relates more particularly to the chemical treatment of an uncured autodeposited coating for the purpose of improving various properties of the cured coating that is subsequently formed from the uncured coating. Most particularly, a major object of this invention is to increase the thermal stability of the chlorine containing polymer coatings formed. It is generally known that polymers of vinylidene chloride, residues of which are the predominant component of a coating resin used in the type of autodeposition bath toward which this invention is directed, have relatively poor thermal stability compared with most other common commercial polymers. One readily noted evidence of this thermal instability is the darkening of the polymers when exposed to heat, and the darkening is normally is well correlated with less overt losses of mechanical strength and resistance to chemical reactions that can severely limit the practical uses of polymers that undergo them. Various additives are known in the general polymer art for increasing the stability of polymers of vinylidene chloride, but all of those tried have been found to impart other unacceptable characteristics to autodeposition baths into which they have been incorporated.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, "parts of", and ratio values are by weight or mass; the term "polymer" includes "oligomer", "copolymer", "terpolymer" and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not necessarily preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word "mole" means "gram mole", and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; and the terms "solution", "soluble", "homogeneous", and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25.degree. C.